244 S1GILLARIEAE. 



point, so that it appears to be in brackets. The organisation of the cortical 

 tissue, which lies at the foundation of these two marks, will have to be con- 

 sidered further on ; but of one thing there can be no doubt, that they are 

 not essentially distinct from the similar structures in Lepidodendrae, and 

 that considering the variations in form to which they are subject in both 

 groups we shall scarcely be able to found a differential character upon them, 

 as Renault l has attempted to do. In addition to these three marks 

 wrinkles more or less strongly marked are to be observed on the surface of 

 the scars, radiating generally from them to the periphery and running either 

 a straight or an undulating course. The strongest of these are apt to 

 attach themselves to the mark and to run in the direction of the median 

 plane. The whole surface of the scar between the wrinkles appears when 

 the preservation is particularly good to be finely shagreened, evidently the 

 impression of the individual cells. In many cases the leaf-scars are as broad 

 as the ribs and extend from one lateral furrow to another, in other cases 

 they are much narrower than the ribs and occupy only a portion of their 

 convex surface. Dawson 2 has attempted to separate the latter forms as 

 belonging to Sigillaria sensu strictiori from the others, which he leaves with 

 Rhytidolepis ; to the latter division belongs also Renault's 3 section Polle- 

 riana, founded on Brongniart's 4 Sigillaria Polleriana, S. Deutschiana and 

 similar forms. Longitudinal striation and furrowing of the dorsal surface 

 of the ribs is a common phenomenon in these broad-ribbed forms. Further 

 the leaf-cushions which coalesce to form a rib differ considerably in length, 

 as has been already said, and the scars therefore are separated by very 

 unequal distances. If the leaf-scars are far apart, the portions of the dorsal 

 surface of the rib which lie between them are very commonly marked with 

 transversal and more or less developed folds and wrinkles, and then if the 

 preservation is sufficiently good a minute pit is often to be seen close above 

 each leaf-scar, and a feathery tuft of delicate markings proceeds from the 

 pit and spreads in every direction. Stur 5 , who was the first to draw proper 

 attention to this peculiarity, sees in it the homologue of his ligular pit in 

 Lepidodendrae, and cites some of the figures from various authors which 

 show this small depression. It should be observed however that the figures 

 in Germar's twenty-fifth table represent forms from the groups Clathraria 

 and Leiodermaria ; a figure from Lesquereux 7 may be added to the list. 

 If the leaf-scars are nearer together, the surface of the cushions necessarily 

 grows smaller. Even in distinct Rhytidolepis-forms we not unfrequently 

 observe indications of transverse boundary-lines of the cushion in the form 



1 Renault (2), vol. iii, Introd. p. 4. a Dawson (8). a Renault (2), vol. i, p. 134. 



4 Brongniart (1), t. 165. 5 Stur (5), p. 293. 6 Brongniart (1), vol. i, t. 144, f. 4 ; 



Goldenberg (1), t. 9, f. 4 ; Germar (1), t. n, f. 2, and t. 25, ff. i, 2. 7 Lesquereux (1), vols. i, 



ii, t. 72, f. ii. 



